Protecting static voltmeters.



Patented Nov. 28, |899.

E. M. HEWLETT.

PRUTECTING STATIC VULTMETERS.

(Application med July 15, 1898.)

Y (No Model.)

T d d Edward M.Hew\etb.

E55 E5. A. lml.

UNITED STATES PATENT OEEICE.

EDWARD M. HEWLETT, OF SCHENECTADY, NEV YORK, ASSIGNOR TO THE GENERALvELECTRIC COMPANY, OF NEW YORK.

PROTECTlNG STATlG VOLTIVIETRS.

SPECIFICATION forming part of Letters Patent No. 637,785, dated November28, 1899.

Application iiled July 15, 1898. Serial No. 685,994. (No model.)

To all whom it may concern: 4

Be it known that I, EDWARD M. HEWLETT, a citizen of the United States,residing at Schenectady,in the county of Schenectady and State of NewYork, have invented certain new and useful Improvements in ProtectingStatic Voltmeters, (Oase No. 631,) of which the following is aspecification.

My present invention concerns electrostatic measuring instruments, thebest example of which is the electrostatic voltmeter, which by amodified construction is also used as a ground detect-or. This is awell-known form of instrument applicable particularly to alterr 5 nating currents and is particularly usefulwith high-voltage circuits; butsuch an instrument must necessarily form a short circuit across theline, except for the air-gap, which must be made very small in order tosecure sensitiveness, and if for any reason the potential should rise sofar as to breakdown the air-barrier a considerable amount of damagemight be done. As the'instrument operates entirely by charge andnormally carries no current, I have found that it can be ecientlyprotected by including between it and the lines resistance in seriessufficient in amount to cut down the current-dow, so that no destructivearc could be maintained between the opposed sur- 3c faces of theinstrument, even if the air-gap should break down. This resistance maybe inductive or non-inductive in character. For some reasons it isdesirable that it should be inductive. Iam not, however, limited in thisregard, and in the accompanying drawing, which shows an embodiment ofthe invention, I have illustrated a resistance of non-inductivecharacter upon each side of the instrument. This is a convenientdisposition; but 4o it is not imperative that the resistance be divided.It is, however, desirable, because it affords protection to both sidesof the circuit in case of an accidental ground.

In the drawing, A B` are the high-potential lines of analternating-current system, and C C are the resistances, each connectedto one of these lines. Each of the resistances is connected to aterminal of the voltmeter D, which is shown as of the usual type,provided 5o with stationary quadrants (as they are commonly called) EE', which are extended surfaces of conducting metal. The vanes F F areattached to the needle f, which moves over the scale G. A counterpoisefis provided, as usual. The scale G is electrically connected to theneedle, so that the vanes F F and the scale form one side of thecircuit, while the quadrants, which are electrically connected together,form the other. The particular type of measuring instrument is, how- 6oever, unimportant, and I have simply described a well-known form forwhich any othermightbesubstituted. Notonlydoes my improvement protectthe instrument against the destructive effect of an accidental arcfollowing the spark passing from one side of the circuit to the other,but it also enables me to safely reduce the distance between the movingand stationary parts of the voltmeter, so that I amplify its readingsand make it very 7o sensitive to small differences of potential.

The operation of the resistance aording the results pointed out may bebriefly described as follows: Since the instrument works by charge andnot by current-liow, so long as the insulation is perfect the potentialbetween its opposed surfaces will be the same irrespective of theresistance that may be in circuit with it. If a spark, however, shouldpuncture the air-insulation, under ordinary 8o circumstances adestructive arc would be formed; but as soon as current begins to flowthe high resistance causes such a drop in voltage that the diierence ofpotential can no longer maintain the arc between the opposed surfaces,and it is extinguished without harm to the apparatus.

Vhat I claim as new, and desire to secure by Letters Patent of theUnited States, is-

l. Thecombinationwithanelectrostaticin- 9o strument, of an impedance inseries relation thereto and to the source of energy, said impedancebeing su flcientlyhigh to prevent the maintenance of an arc if thepotential rise sufliciently to leap the air-gap between the l fixed aridmovable members.

2. The combination with an electrostatic instrument, of a resistance inseries relation to said instrument and a source of electric energy, saidresistance being suihciently high Ioo 2A y v 637,785

' to prevent the maintenance of an are if the eurrent-lomexjen iftheinstrurnent' breaks ro potential should rise high enough-toleapthedo:v vn.

air-gap between the fixed and movable me'm- In vwitness whereof Inh-:ivehereunto set my bers. l Y l hand this 12th clay of July, 1898.

3. The combination with an alternatingy current circuit and ailelectrostatic instru- EDWARD HEWLETT" ment connected across the circuit,of a resist- Witnesses: w, A ance between each main and the instrument fB. B. HULL, v

sufficient in lamount to substantiallyprevent G. HYNES.

